The overall goal of this proposed research is to investigate the development of the brain-pituitary-adrenal-axis in the mammalian fetus. This includes determining: 1) if the system is regulated by a negative feedback relationship during gestation, 2) if responsiveness of the system to negative feedback regulation changes during the last half of gestation, and 3) if the origin of the negative feedback signal changes during development. Our aim is also to ascertain if responsiveness of the system to stimulation is modulated by catecholamines and vasopressin since changes in the relative importance of these agents may explain maturational changes in ability of the pituitary-adrenal axis to respond to stress. The chronically cannulated lamb fetus is the animal model chosen for study. With this model it is possible to study brain-pituitary-adrenal relationships in the absence of complications caused by the presence of anesthesia and surgical trauma, both of which are inherent in experiments on acutely prepared animals. The information derived from these proposed studies will further our understanding of the physiological regulation of an endocrine system necessary for preparing the fetus birth, and, in some species, necessary for the initiation of labor. Increasing our understanding of the ontogeny of the control of this system is essential for furthering our knowledge of developmental processes which culminate in the birth of a healthy infant.